Field of the Invention
The present invention relates to a vibration motor controller, and a lens apparatus and an image pickup apparatus that include the vibration motor controller.
Description of the Related Art
There have been proposed various methods for controlling a vibration motor having a vibrator formed of a metal elastic body or the like to which an electromechanical energy transducer (piezoelectric element or electrostrictive element) is bonded, and a contactor that comes in pressure-contact with the vibrator. Japanese Application Laid-Open No. 2011-067035 discloses a technique of extending a dynamic range of the velocity of the vibration motor by performing phase difference control to change the phase difference with the frequency fixed, and then performing frequency control to change the frequency with the phase difference fixed when the phase difference reaches a prescribed value.
The technique disclosed in Japanese Application Laid-Open No. 2011-067035 is a technique that enables use of a wide dynamic range of the driving velocity of the vibration motor. However, in the frequency control after switching from the phase difference control, even if the frequency is raised or lowered by a certain amount, the driving velocity of the vibration motor does not increase or decrease correspondingly linearly, which lowers the controllability. Hence, in order to reduce an adverse effect of such nonlinearity of the driving velocity with respect to the frequency in the frequency control domain, the frequency at phase difference control may be set to a lower frequency. In this case, however, the controllability at a low velocity region tends to be lowered. This is because a velocity change with a frequency change is large in a frequency region lower than the resonance frequency in the frequency-velocity curve, and because the resonance frequency in the frequency-velocity curve with a small phase difference is higher than that in the frequency-velocity curve with a large phase difference. In sum, if the frequency at the phase difference control is set to a higher frequency, the controllability is lowered due to the adverse effect of the nonlinearity in the frequency control domain; if the frequency at the phase difference control is set to a lower frequency to reduce the adverse effect of the nonlinearity in the frequency control domain, the vibration motor unstably operates due to control in accordance with the frequency-velocity curve in a frequency region lower than the resonance frequency. Hence, it has been difficult to achieve the controllability of the vibration motor and the stability of the vibration motor with a small phase difference at the same time.